Primary peritoneal low-grade serous carcinoma forming a mass in the colon mimicking a colonic primary carcinoma: a case report.

Primary carcinomas of Müllerian origin involving the colon is not an uncommon phenomenon, with most cases reportedly associated with endometriosis. On the other hand, a primary peritoneal low-grade serous carcinoma presenting as a dominant mass in the colon and causing clinical symptoms mimicking a primary colonic carcinoma has not been reported in the literature to the best of the authors' knowledge. A case of a 66-year-old female patient who presented clinically with rectal bleeding and a rectosigmoid mass is described. The final histologic examination revealed a peritoneal low-grade serous carcinoma forming a dominant mass in the rectosigmoid colon. Of particular interest was a microscopic spectrum of serous epithelial proliferation in the peritoneal cavity and lymph nodes with morphologic features reminiscent of non-invasive and invasive implants in ovarian borderline serous tumors, which most likely denoted the precursors of the tumor in the colon.